Our invention relates to amplifiers in general and, in particular, to a broad-band amplifier system having essentially flat response over a frequency range of, say, from zero (direct current) to the order of gigahertz (GHz). The amplifier system of our invention is particularly well adapted for use with oscilloscopes or oscillographs, among other applications.
An amplifier system for wide-band cathode-ray oscilloscopes normally consists of a preamplifier and a main amplifier. The preamplifier has provisions for gain control, polarity switching, etc. The main amplifier, on the other hand, offers high gain to amplify the signal sufficiently to drive the cathode-ray tube. The input impedance of such oscilloscopes is usually set at one megaohm, but that of 50 ohms is more common for sampling oscilloscopes and wide-band oscilloscopes covering frequencies up to several hundred megahertz (MHz) or more. Such wide-band input signals are usually of such low levels, however, that the gains of the oscilloscope amplifiers available today are mostly insufficient to amplify them to an extent required for their waveform observation. The need has arisen for the added use of wide-band, high-gain amplifiers, particularly for the measurement of electrical quantities in the research and development of optical communications technology and broad-band semiconductor devices.
Wide-band, high-gain amplifiers present an assortment of problems that must be remedied to establish their true utility. These are degradation in the signal-to-noise ratio, oscillation, drift, and the deterioration of performance characteristics, particularly in the case of the multistage connection of wide-band amplifiers.
In the art of optical communications, for example, there has been awaited the advent of wide-band, low-noise, low-drift amplifiers for the waveform observation of optical signals of over 1 Gb/s. The observation of optical signals requires, before amplification, the translation of extremely small light energy into electric signals by means of wide-band, low-noise photodetectors such as p-i-n photodiodes and avalanche photodiodes. This makes it difficult to control the gain of the amplifier system in the manners common with usual oscilloscopes. A sufficiently broad frequency band and high signal-to-noise ratio have been difficult to obtain through gain control by the amplifier itself, as with one- or 10-megaohm-input amplifiers, or in combination with an attenuator. It has also been not easy to make gain control over a wide range without impairment of frequency response.
We are aware of a variety of wide-band amplifiers heretofore suggested and used for the purposes under consideration (two typical ones illustrated in FIGS. 1 to 3 of the drawings attached hereto). Each such conventional device has one or more of such drawbacks as distortion, uneven gain, drift, and expensiveness, as will be later detailed.